The quality of paper produced from cellulose fibers (i.e. wood pulp or the paper produced by the recycling of paper made from wood pulp) is often judged by its opacity. Paper producers have long sought to improve these vital characteristics so that an enhanced paper may be obtained.
This and other desired characteristics have been obtained in the past by supplying the pulp slurry of cellulose fibers or furnish with additives prior to the slurry entering the papermaking machine. Various additives are well known in the art. For example, titanium dioxide powder is known to be an excellent whitener. Titanium dioxide, however, is among the most expensive materials that may be added to the slurry. Thus, despite the effectiveness of such material as a brightener, its use is limited and satisfactory replacements have been needed.
Kaolin clay has also been used as a filler in paper to improve brightness in the ultimate product. Generally, the kaolin clay is calcined and then suspended in an aqueous solution prior to being added to the furnish. The clay must be continuously agitated prior to entering the slurry or the solid particles begin to form sediment at the bottoms of the clay holding tanks. Although kaolin clay provides brightness, as well as opacity to the finished paper product, the relative difficulty of adding it to the slurry results in a less than excellent additive.
When clay is added to the pulp slurry, the slurry needs additional chemicals. A retention aid is necessary to retain the clay in the sheet, which will add extra cost to the sheet. Adding clay to the slurry will also have an adverse effect on drying the sheet of paper. The paper maker will slow the paper machine down to maximize the drying to make sure the sheet is dried, which will increase the cost of the sheet. The clay also increases wear on the paper machine. This wear shows up in shorter life for some of the pans of the paper machine. The wire, felt, doctor blade and refiners especially, show wear when clay is used. With the increased abrasiveness of the clay down time is longer and more frequent.
Hydrated aluminum silicate has also been employed as a clay substitute in the papermaking process. It has properties similar to kaolin clay and, thus, results in the same disadvantages when used to make paper.
Many compositions have been added to the slurry in an attempt to size the paper, i.e. render the paper water repellent. Most known sizes, such as those disclosed in U.S. Pat. No. 2,142,986 to Arnold, Jr. and U.S. Pat. No. 3,096,232 to Chapman, employ a type of wax. For example, Arnold, Jr. discloses that an emulsion of wax in a solution of deacetylated chitin, paraffin waxes, Japan wax, carnauba wax, higher aliphatic alcohols, or synthetic waxes may be employed as the waterproofing agent in a sizing composition. A softening agent such as aliphatic alcohols containing 12 to 20 carbons is also present in the composition of Arnold, Jr. Chapman discloses the use of paraffin waxes or water-insoluble derivatives of resins for producing aqueous wax emulsions with cationic modified starches.
In U.S. Pat. Nos. 5,296,024 and 5,292,363 a papermaking composition is disclosed for enhancing opaqueness comprising the reaction product of fatty acid and diamine. This composition is effective although further improvements are desired for strength of the paper, and coefficient of friction.